There is significant daily variation in the total amount of power required from a baseload power plant. However, it would be costly and wasteful to provide for peak demands of short duration with more baseload power plant machinery.
In the past, power utilities have addressed the problem of providing additional non-baseload peaking power in an economic manner through the use of simple cycle gas turbines ("SCGTs"). Simple cycle gas turbines are state-of-the-art turbomachinery designed for peaking duty operation. Their main elements are an air compressor, a turbine, and a combustor. To meet consumer demand, SCGTs are coupled to electric generators to provide additional power to baseload power plants during peak loads.
Most recently, power utilities have addressed this problem of supplying peaking power in a cost effective manner by use of Compressed Air Energy Storage ("CAES") systems. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear baseload plants by compressing air into storage devices such as underground caverns or reservoirs. Underground storage can be developed in hard rock, bedded salt, salt dome, or aquifer media.
Following off-peak storage, the compressed air is withdrawn from storage, heated with fuel, and expanded through expanders, i.e., turbines, to provide needed peaking or intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as about two-thirds compared with conventional gas turbines. Under these circumstances, utilization of non-peak energy for subsequent production of peak energy is clearly desirable, especially since non-peak energy can often be obtained for far less than the cost of obtaining peak energy.
Compressors and turbines in CAES plants are each connected to a synchronous electrical generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during appropriate selected time periods.
During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme, the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to storage.
During peak/intermediate periods, with the turbine clutch engaged, compressed air is withdrawn from storage and provided to a combustor. In the combustor, the compressed air oxidizes a fuel, and the combustion by-products are charged to a turbine, which generates power by driving the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES heat rate, waste heat from a low pressure turbine exhaust is used to pre-heat high pressure turbine inlet air in a recuperator.
For a more complete discussion of CAES systems, see M. Nakhamkin, et al. "Compressed Air Energy Storage: Plant Integration, Turbomachinery Development," ASME International Gas Turbine Symposium and Exhibition, Beijing, Peoples' Republic of China, 1985 and M. Nakhamkin, et al. "Compressed Air Energy Storage (CAES): Overview, Performance and Cost Data for 25 MW to 220 MW Plants", Joint Power Generation Conference, Toronto, Canada 1984, both of which are hereby incorporated by reference.
Examples of CAES systems are disclosed in U.S. Pat. No. 4,100,745 to Gyarmathy et al., U.S. Pat. No. 4,593,202 to Dickinson, and U.S. Pat. No. 4,872,307 to Nakhamkin. These systems are directed to electrical power storage and generation alone.
In order to maximize the efficiency of a CAES system, it is desirable to preheat gas released from the storage cavern by heat-exchanging contact with that gas after expansion. See U.S. Pat. No. 4,936,098 to Nakhamkin, U.S. Pat. No. 4,150,547 to Hobson, and U.S. Pat. No. 4,523,432 to Frutschi.
Another heat recovery technique involves the use of heat generated during compression to warm gas released from storage.
In U.S. Pat. No. 4,150,547 to Hobson, compressed air entering an air storage cavern first passes through a heat storage cavern to recover and store (in a heat storage medium) heat in the compressed air. When air is released from the storage cavern, it again passes through the heat storage cavern, where it is heated.
In U.S. Pat. No. 4,347,706 to Drost, boiler water used to drive steam turbines for compressing air is heated in the compressor innercoolers and aftercoolers, and excess energy from the boiler is stored in a thermal storage unit. Air released from storage is passed through preheaters prior to expansion where heat is provided by the thermal storage unit.
U.S. Pat. No. 4,523,432 to Frutschi discloses a CAES system where heat is recovered from the compressor innercoolers and is stored in a heat storage device by passing water between the innercooler and that device. Compressed air released from storage passes through an exchanger where it is heated by hot water from the heat storage device.
Despite the continued development of heat recovery systems for CAES systems, the need to improve the thermal efficiency and match the thermal cycle to demand for such systems remains.